All publications mentioned throughout this application are fully incorporated herein by reference, including all references cited therein.
Tissues in the body contain spaced capillaries that provide conduits for the convective transport of nutrients and waste products to and from the tissues. However, cell constructs that are developed ex-vivo usually lack the vascular network that exists in normal vascularized tissues. Hence, the gas and nutrient supply to and from ex-vivo cell constructs depends solely on mass transport (e.g. diffusion) of the growth medium.
It is well known that bioreactors that are employed to cultivate cell constructs are designed to improve the mass transport of nutrients and other products within the growing tissues. Various kinds of bioreactors make use of different patterns of fluid dynamics and vessel geometry. Ideally, bioreactors must allow for control over the physicochemical environment (e.g. pO2, pH, pCO2, shear stress, etc.), allow for aseptic feeding and sampling in order to follow tissue development and maximize the use of automated processing steps in order to increase reproducibility.
Standard bioreactor technologies that are known in the art are well suited for addressing the many issues involved in 2-D cell expansion, but have limitations when used for other tissue engineering applications. For example, the cultivation of 3-D tissue constructs places large demands on the mass transport requirement (e.g. nutrient distribution). Furthermore, it is sometimes necessary to simultaneously culture multiple cell types for a certain application, which would require more complex bioreactor designs.
The present inventors have previously reported the cultivation of cardiomyocytes constructs in rotating cell culture systems (RCCS), which were developed by NASA (see Shachar M., et al., (2003) Ex-vivo engineering of cardiac muscle: Cultivation in rotating vessels. Proceedings of EMCC-Bioengineering). The operating principles of the RCCS are solid body rotation about a horizontal axis, which is characterized by extremely low fluid shear stress, and oxygenation by active or passive diffusion of dissolvable gasses from the reactor chamber, thereby yielding a vessel devoid of gas bubbles and gas/fluid interfaces. The present inventors showed that pO2, pH and pCO2 were maintained in the RCCS to a better extent, and aerobic respiration was allowed for a larger number of cells, as compared to performance in a static vessel. Cultivation of cardiac cell constructs in RCCS produced engineered cardiac tissues with improved cellularity, cell metabolism and expression of muscle specific markers.
Although the RCCS provided a nearly homogeneous ex-vivo environment for the 3-D cell constructs, the extent of medium perfusion into the core of the cultivated tissue was still limited due to the absence of a capillary network in the developing tissue. As a result, the cells at the center of the 3-D cultivated tissues did not benefit from the external dynamic fluid.
It is therefore an aim of the present invention to provide a bioreactor system that overcomes the problems involved in previous bioreactor systems, specifically poor mass transfer.
It is another aim of the present invention to provide a bioreactor system that enhances mass transport of a desired medium into a developing tissue.
It is another aim of the present invention to provide a bioreactor system which pumps a growth medium directly through 3-D cell-seeded scaffolds.
It is another aim of the present invention to provide a bioreactor system which pumps a growth medium through a 3-D cell-seeded scaffold in a similar manner to the pumping activity of a heart.
Other aims and advantages of the present invention will become apparent as the description proceeds.